Figured I'd fill you in. It is expected that Henson will quit baseball after this year for football, but until the end of the year, he'll be playing baseball.
Eric Almonte, the Yanks' backup SS (for the injured Jeter), got hurt- I think it was his calf. So, the Yanks called up Henson as a backup infielder. According to the Star Ledger, he is unlikely to get much or even any playing time in his time up in the majors.
For those interested in his contract situation- he originally signed a 6-year $17 million deal in 2001. Right now, he has 3 years and $12 million left on his contract. Also according to the Star Ledger, if he quits baseball he will most likely lose the rest of the money in his deal, meaning he won't get the $12 million left on his contract.
Hope this helps shed light on the situation